During the past two decades, governmental fuel conservation and safety mandates along with global competition and environmental concerns have prompted the automotive industry to design lighter vehicles for reduced fuel consumption and reduced manufacturing costs, while improving the overall structure of the vehicle for occupant safety. A relatively new process known as tailor welded blank (hereafter TWB) forming has been developed in an attempt to meet these needs. The TWB forming process replaces the traditional forming-welding sequential process with a welding-forming sequential process. The TWB forming process involves joining various metal sheet sections (e.g. steel sheets) having different properties, such as thickness, strength, etc., into a single welded blank for subsequent forming operation to shape. Therefore, optimum material properties can be located precisely within the formed part where needed for a particular service application. For example, usually, thicker and/or stronger sheet material is used at locations that previously required reinforcement parts. The potential benefits of the TWB forming process include fewer parts, fewer forming dies, fewer spot welds, less material input, and better utilization of sheet metal. Consequently, use of TWB forming processes will result in weight reduction, improved structural integrity, reduced scrap, and most likely lower manufacturing costs and improved dimensional accuracy.
A critical aspect in practicing the TWB forming process concerns movement of the weld line which connects two different sheet materials having different material thickness and/or different strengths. For example, the sheet of weaker material tends to deform more than the sheet of the stronger material, and therefore, the weld line will move toward the stronger material during deformation of the tailor welded blank to shape. Such weld line movement contributes additional potential for tearing, wrinkling, die wear, and distortion and parts dimensional variations when compared to conventional one-blank forming process.
In prior tailor welded blank forming operations, a blank comprising welded sheets is placed between a binder and a die. In the first step of the forming operation, a binder force is applied to the blank periphery to provide a restraining force to the blank through the frictional force from the binder interface plus bending resistance if drawbeads are used. In the second step of the forming operation, a punch is displaced to draw the blank under the binder into the forming zone and to plastically deform the blank into desired shape. The optimal placement of the weld line(s) is selected mainly to meet requirements of safety and integrity of the formed product. Unfortunately, the forming difficulty, in particular weld line movement, introduced by the TWB forming process has resulted in usage of extra thicker/stronger material, thereby reducing the extent of weight and material savings achievable. To overcome this disadvantage, workers have attempted to apply a lower restraining force to the side of the stronger material section of the blank than that applied to the weaker material section of the blank in a conventional one-die forming operation.
For example, different restraining forces on the stronger versus weaker material sections of the tailor welded blank can be applied in a manner described by Siegert et al. in "Closed Loop Binder Force System", SAE paper 960824, 1996, wherein a segmented binder having several individually controllable binders is used in lieu of the traditonal one-piece binder. However, the segmented binder described is not advantageous in that there is a reduction of the rigidity of the overall binder, especially when many binder segments are used, and in that there is a possibility for shear of the material across binder segments. The pressure discontinuities existing across the binder segments may cause earlier failure in the tailor welded blank, since the weld line cannot support the shear deformation as well as the blank base materials.
Another approach which has been described by Jimma and Sekine in "Effect of Rigidity on Blank Accuracy of Electronic Machine Parts" in Annals of the CIRP, pages 319 to 322, 1992, and by Siegert et al. in U.S. Pat. No. 5,138,857 (also see "CNC Hydraulic Multipoint Blankholder System for Sheet Metal Forming Presses", Annals of the CIRP, Volume 42(1), pages 319 to 322, 1993, involves use of a deformable/elastic binder where a relatively thin binder (e.g. about 2 inches in thickness) is set on multiple pins. Each pin is driven by a hydraulic cylinder that generates an adjustable supporting pressure. Due to the elastic compliance of the thin binder, an uneven pressure distribution can be generated and can affect material flow during blank deformation. The use of a deformable/elastic binder in this manner is not advantageous in that controllability on the magnitude of the pressure difference over the entire binder is limited by inherent flexibility of the binder and in that the thin binder may have a relatively low durablity in high volume production. Also see the Shulkin et al. article "Elastic deflections of the blank holder in deep drawing of sheet metal" in Journal of Materials Processing Technology, pages 34 to 40, 1996.
Still another approach to mitigating weld line movement involves use of different drawbead heights/shapes or binder radii on two opposite sides of the weld line so that the stonger material section of the blank can flow into the deformation zone more easily that the weaker material section of the blank. These techniques are disadvantageous in that control is difficult and there is more material waste.
A common feature of the aforementioned approaches to TWB forming is that movement of the weld line(s) of the blank is determined by the binder design and/or applied peripheral blank restaining pressure or force which alters the amount of material drawn into the die cavity. Another common feature is that attempted control of weld line movement occurs remote from the weld line(s) and deformation zone, thus inherently limiting the extent to which weld line movement can be controlled.
Furthermore, the aforementioned approaches may not be effective when a nonlinear (curvilinear) weld path and/or multiple weld lines are present in the tailor welded blank. For example, the lower binder force might be beneficial to one weld line, or the material flow in in one direction, but meanwhile could be undesirable for another weld line in another direction.
An object of the present invention is to provide method and apparatus for locally holding a weld line(s) of a tailor welded blank during at least initial forming thereof (where it has been discovered that most of the weld line movement occurs) so as to control and limit movement of the weld line in a manner that reduces movement of the weld line and overcomes the aforementioned disadvantages of the TWB forming processes described hereabove.